It's a complex question. To be stunningly reductive: the architectures are simply quite different. We hook into the network stack in different places, we have different sandbox models and constraints (Win XP for instance), etc.

Also note that the timing is fairly important: Chromium was quite far along with our implementation when WebKit2 was announced, and rather than iterating on the solution we'd proposed and run with, Apple created its own framework. That had advantages and disadvantages.

More generally, I'd point to the Content layer as a better integration point: Opera, for instance, is building on top of our multi-process architecture successfully. Chromium Embedded Framework (https://code.google.com/p/chromiumembedded/) is another example of how other projects can leverage the work we've done.